An industrial process sensor is typically a transducer that responds to a measurand with a sensing element and converts a variable to a standardized transmission signal, e.g., an electrical or optical signal, that is a function of the measurand. Industrial process sensors utilize transducers that include flow measurements of an industrial process such as slurries, liquids, vapors and gasses in refinery, chemical, paper, pulp, petroleum, gas, pharmaceutical, food, mining, minerals and other fluid processing plants. Industrial process sensors are often placed in or near the process fluids, or in field applications. Often, these field applications are subject to harsh and varying environmental conditions that provide challenges for designers of such sensors. Flow measurement is one of the largest segments of the industrial sensing and instrumentation market. Industries in which flow measurement is prevalent includes petroleum, chemical, pulp, paper, food, and mining and minerals.
In many industries it is desirable to measure the flow rate of a multiphase fluid. In many industries such as refinery, chemical, paper, pulp, petroleum, gas, pharmaceutical, food, mining, minerals or comparable industries, for example, it is desirable to measure the flow rate of multiphase fluids, especially fluids having three phases, such as a constituent, water and gas. It is known also to measure the flow rate of certain fluids (one or more liquids and/or gases) in a pipe using cross-correlation flow meters. Such meters measure an element of the flow that moves or convects with (or is related to) the fluid flow (or a group of fluid particles). The meter measures this element at two locations separated by a known distance along the flow path and then calculates the time for such element to move between the two locations. The time delay is determined by a cross-correlation of the two measured signals. A velocity is then determined by the distance between the measurements divided by the time delay. The flow velocity is then related to the flow rate by calibration.
One such cross-correlation meter that measures flow rate in a multiphase flow is described in U.S. Pat. No. 5,591,922, entitled “Method and Apparatus for Measuring Multiphase Flow”, to Segeral et al, issued Jan. 7, 1997. In that case, a pair of venturis is located a predetermined distance apart which induce a change in flow speed through the venturi and a resulting pressure difference (or delta-P) across each venturi, which are measured. The delta-P pressure signals measured at each venturi are cross-correlated to determine the time delay which is indicative of the total volume flow rate. However, such a technique requires a change in the flow properties (e.g., flow velocity or density) at the two measurement points to make the measurement. Also, the delta-P is generated with an area contraction or constriction, and is not a naturally occurring observable characteristic of the fluid.
Other flowmeters of the prior art include turbine, vortex, electromagnetic and venturi and all have drawbacks and deficiencies solved by the flowmeter of the present invention. For instance all require a high level of maintenance and need to be removed from the process line wherein the operators need to shut down the manufacturing process. The flowmeters of the prior art require electrical wiring and power that requires enormous cost, safety issues and sometimes miles of wires. Many of the prior art meters use moving parts, such as turbines or diaphragms. Also, prior art flowmeters such as vortex, turbine and venturi types use obstructions in the flow path that disrupt the flow to varying degrees. In addition sediment, gumming, plugging, corrosion, and erosion of certain features of the sensing region of the meter can affect the accuracy of the flowmeter.
In particular, electromagnetic flowmeters are prone to problems caused by poor process grounding, and specialized sleeves that are prone to damage. Process noise is a problem and can be caused by slurries, high consistency pulp stock, or upstream chemical additions. Such process noise can lead to inaccurate flow measurement in these types of prior art flowmeters. In addition, process liquids must have a minimum conductivity that all but eliminates these meters from uses where the fluid is a hydrocarbon. The accuracy and sensitivity can be affected by the length of cabling for remote transmitters and can be adversely influenced by proximity to other electrical devices
Another flow meter of the prior art includes a friction flowmeter such as that set forth in U.S. Pat. No. 6,253,624, titled “Friction Flowmeter” wherein a transducer determines the pressure drop of a fluid flowing along a pipe. The device determines the flow rate of the fluid based on the pressure drop for a given friction factor of the inside surface of the pipe. Such a device requires external knowledge of various parameters of the fluid, such as density. In addition, with certain applications the surface of the pipe, and the friction factor thereby, would change over time and decrease the accuracy of the meter in predicting fluid flow rates.
Typical electronic, or other, transducers of the prior art often cannot be placed in industrial process environments due to sensitivity to electromagnetic interference, radiation, heat, corrosion, fire, explosion or other environmental factors. It is for these reasons that fiber optic based sensors are being incorporated into industrial process control environments in increasing number.